1. Field of the Invention
The present invention relates to an antenna device, particularly to an antenna device formed on a window glass panel of a motor vehicle.
2. Related Art
In the case of an antenna having a frequency band of GHz or more for a motor vehicle, it is generally desired that the entire structure of the antenna is formed on the surface of a window glass panel considering the size of the antenna. In this case, it is difficult to provide a through hole in a glass panel at a feeding point of the antenna, so that the antenna must be formed on one surface of a window glass panel. An antenna formed on one surface of a window glass panel is referred to as a coplanar antenna. As a coplanar antenna, the antenna disclosed in Japanese Patent Publication No. 08-148921 and PCT International Publication No. WO3/105278 is known.
In order to give a directivity to the antenna disclosed above-described Publications, it is conceivable that a reflector disclosed in PCT International Publication No. WO2004/004070 is provided. In this case, it is preferable that a box-shaped electronic circuit unit comprising integrally an electronic circuit including an amplifier for amplifying a received signal to increase the performance of an antenna, a reflector, and a housing, and having an opening is provided detachably in such a way that the unit covers a planar antenna formed on a window glass panel through a base plate without DC coupling between the antenna and the electronic circuit unit.
The reason why the electronic circuit unit is provided detachably is to make an exchange thereof easy if the unit is failed. In this case, a coaxial cable is used for connecting between the feeding points of the antenna and the amplifier. However, it is undesirable that the coaxial cable is unnecessarily extended of the stage previous to the amplifier, because there is no merit to provide the amplifier and the S/N ratio is degraded, if a signal attenuation is large. Accordingly, the amplifier is provided in the electronic circuit unit near to the antenna as described above.
Even if the length of the coaxial cable is short, the following problem is caused. That is, a coaxial cable is an unbalanced circuit, so that a common mode current through a central conductor and outer conductor of the coaxial cable is caused in addition to a normal mode current through the central conductor and outer conductor of the coaxial cable. This is due to the fact that a ground antenna element to which the outer conductor of the coaxial cable is connected and a metal plate (a ground plane) which constitutes the outer wall of the electronic circuit unit are capacitively connected to form a pseudo signal path between the outer conductor of the coaxial cable and the ground plane, thereby the common mode current which is originally unnecessary is induced.
In FIG. 1, there is shown the condition in which a common mode current flows through a coaxial cable 12 connected to a balanced antenna. When such a common mode current flows, the common mode current is added to the original normal mode current, so that the exciting current distribution of the antenna and coaxial cable is put out of order, resulting in a remarkable degradation of the reception performance of the antenna. The common mode current is varied depending on the wiring state and the length of the coaxial cable, so that the reception performance of the antenna is also varied. Accordingly, the common mode current through the coaxial cable is extremely undesirable current.